A portable, collapsible child booster seat for a vehicle comprises a collapsible seat member that removably rests on a vehicle seat with collapsible arm rests extending upwardly from opposite sides of the seat member. The seat member includes an upper seating surface supported on a collapsible scissors frame seat base. The seat base is raisable to a deployed position and collapsible to a storage position. The arm rests comprise horizontal arms pivotably mounted on raisable legs. The arm rests have a deployed position, wherein the legs extend upwardly from the seat base and the arms extend forwardly, and a collapsed position, wherein the legs and arms are in a generally horizontal position adjacent the sides of the seat base. The arm rests and seat base are operably interconnected, such that raising or lowering the arm rests between their deployed and collapsed positions causes deployment and collapse of the seat base.